Accompanied with the rapid development of the modern industry, the confliction between supplying and demanding of energy has become increasingly acute. China is a major energy consumer and meanwhile a major country of energy shortage with an urgent desire for searching replaceable energy sources. Ethanol is a clean energy source with a good mutual solubility which can be used as blending component added into gasoline, to partially replace gasoline and improve the octane number and the oxygen content of gasoline. It can also promote gasoline burning sufficiently and decrease the emission of carbon monoxide and hydrocarbons in vehicle exhaust. As a partial replacement of vehicle fuel, ethanol may make the vehicle fuel in China more diversified. Currently, in China cereals, especially corns, has mostly been used as a raw material to manufacture fuel ethanol. China has become the third largest country of ethanol producing and consuming, after Brazil and America. However, according to Chinese national condition, there are many unfavorable factors using cereals as raw material to produce ethanol. In the future, non-cereal routes for producing ethanol will be developed preferably in China.
Started with coal resources, producing ethanol via syngas is an important direction to develope coal chemical engineering industry in China with a broad market prospect. It has great strategic meanings and far-reaching impacts on clean utilization of coal resources, relieving the pressure of lacking oil resources and enhancing energy security in our country. Currently, there are mainly two process routes of preparing ethanol from coal, one of which is preparing ethanol from syngas directly. However, a precious metal, rhodium, is needed to serve as the catalyst in this route, so the cost of the catalyst is relatively high. Moreover, the output of rhodium is limited. The other route is preparing ethanol from syngas through hydrogenation of acetic acid, in which acetic acid is preformed by liquid phase methanol carbonylation from the syngas, and then converts to ethanol by hydrogenation. The second route is mature, but the device used in this route needed to be made of special alloy which is anticorrosive, so the cost is high.
Using dimethyl ether as raw material, methyl acetate can be directly synthetized by carbonylation of dimethyl ether, and methyl acetate can be hydrogenated to ethanol. Although the route is still in research stage, it is a brand new route with great application prospect. In 1983, Fujimoto (Appl Catal 1983, 7 (3), 361-368) used Ni/Ac as catalyst to carry out a gas-solid phase reaction of dimethyl ether carbonylation. It was discovered that dimethyl ether can react with CO to generate methyl acetate when the molar ratio of CO/DME is in a range from 2.4 to 4, with selectivity in a range from 80% to 92% and the highest yield of 20%. In 1994, Wegman (J Chem Soc Chem Comm 1994, (8), 947-948) carried out a dimethyl ether carbonylation reaction using heteropolyacid RhW12PO4/SiO2 as the catalyst. The yield of methyl acetate was 16% and nearly no other side products were generated. In 2002, Russian researchers, Volkova and her colleagues (Catalysis Letters 2002, 80 (3-4), 175-179) used a cesium phosphotungstate modified Rh as catalyst to carry out the carbonylation reaction of dimethyl ether and the reaction rate is an order of magnitude higher than the Wegman's reaction using RhW12PO4/SiO2 as catalyst.
In 2006, Enrique Iglesia's research group in Berkeley (Angew. Chem, Int. Ed. 45(2006) 10, 1617-1620, J. Catal. 245 (2007) 110, J. Am. Chem. Soc. 129 (2007) 4919) carried out dimethyl ether carbonylation on the molecular sieves with 8 membered ring and 12 membered ring or 10 membered ring, such as Mordenite and Ferrierite. As a result, it was considered that the carbonylation reaction happenes on the B acid active center of 8 membered ring. The selectivity of methyl acetate was quite good, reaching 99%, but the activity of dimethyl ether carbonylation is very low.
American application US2007238897 disclosed that using molecular sieves with 8 membered ring pore structure, such as MOR, FER and OFF, as catalyst for the carbonylation of ethers, the pore size of the 8 membered ring should be larger than 0.25×0.36 nm. Using mordenite as catalyst under the reaction conditions of 165° C. and 1 MPa, a space-time yield of 0.163-MeOAc(g-Cat.h)−1 was achieved. WO2008132450A1 (2008) disclosed a MOR catalyst modified by copper and silver, whose performance is obviously better than unmodified MOR catalyst, on reaction conditions of hydrogen atmosphere and temperature ranging from 250° C. to 350° C. WO2009081099A1 disclosed that the carbonylation performance of MOR catalyst with smaller grains is better than MOR catalyst with bigger grains. WO2010130972A2 disclosed an MOR catalyst treated by desilication and dealuminzation, and the activity and the reaction stability of the MOR catalyst can be significantly enhanced by an optimized combination of acid treatment and alkali treatment. Moreover, CN103896769A disclosed a method for preparing methyl acetate by carbonylation of dimethyl ether, in which mordenite and/or ferrierite were used as the catalyst. CN101903325A disclosed a carbonylation process of preparing acetic acid and/or methyl acetate in which the molecular sieves with MOR framework structure were used as the catalyst. CN101687759A disclosed a method for carbonylating methyl ether in which zeolites with MOR, FER, OFF or GME framework structures were used, specifically such as mordenite, ferrierite, offretite and gmelinite. Wang Donghui (“Application of a cocrystallization molecular sieve catalyst in preparing methyl acetate by carbonylation of dimethyl ether”, Chemical Production and Techniques (2013), No. 3, Vol 20, 14-18) disclosed an application of a cocrystallization molecular sieve catalyst in preparing methyl acetate by carbonylation of dimethyl ether, in which the catalyst was a cocrystallization molecular sieve containing 2 phases of BEA/MOR. And cocrystallization molecular sieve containing 2 phases of EMT/FAU was mentioned in the first paragraph, without being used for carbonylation of dimethyl ether to methyl acetate. CN102950018A disclosed the reaction data of dimethyl ether carbnylation on a cocrystallization molecular sieve of rare earth ZSM-35/MOR. The results show that the activity and stability of cocrystallization molecular sieve was significantly better than ZSM-35 catalyst, and the stability of cocrystallization molecular sieve was significantly better than MOR catalyst. Xu Longya and his colleagues (RSC Adv. 2013, 3:16549-16557) also reported the reaction properties of ZSM-35 treated by alkali in carbonylation of dimethyl ether. The results show that after being treated by alkali, ZSM-35 has an apparent mesoporous structure, enhancing the diffusion effects of reactants and products on the catalyst, and the stability and activity of the catalyst was improved.
In CN101613274A, pyridine organic amines were used to modify mordenite molecular sieve catalyst, and it was discovered that the modification of molecular sieve can dramatically enhance the stability of catalyst. The percent conversion of dimethyl ether was in a range from 10% to 60%, and the selectivity of methyl acetate was over 99%. Moreover, the activity of the catalyst remained stable after reacting for 48 h. Shen Wenjie (Catal. Lett. 2010, 139:33-37) and his colleagues made a research on preparing methyl acetate by carbonylation of dimethyl ether, comparing the reaction activity on MOR and ZSM-35 catalyst. It was discovered that ZSM-35 molecular sieve has better reaction stability and products selectivity, and under the reaction conditions of 250 □, 1 MPa, DME/CO/N2/He=5/50/2.5/42.5 and 12.5 mL/min, the percent conversion of dimethyl ether could reach 11%, and the selectivity of methyl acetate could reach 96%.
The above references has disclosed a lot of research results on dimethyl ether carbonylation, and research on the catalyst has mainly focused on MOR, FER, and the like with a structure of 8 membered ring. In the results reported publicly, those catalysts are very easy to become inactivated with catalyst life of less than 100 h. And additionally, the reaction results cannot meet the requirement of industrial production.